Bump minimal version required for GNUTLS.

Part of eng/toolchain/aws#31

Author: TurboGit

readme.txt

@@ -119,7 +119,7 @@ LibreSSL (optional):
 GNUTLS (optional):
 
    You need to install manually the development libraries. AWS needs at
-   least version 3.2.4 of GNUTLS.
+   least version 3.4.6 of GNUTLS.
 
 OpenLDAP library (optional) :